The present invention relates to a radio communication apparatus and a radio communication method used in a radio communication system such as a portable telephone and a car telephone.
In a radio communication, a phase of a received signal rotates against a phase of the transmitted signal by frequency offset. It is thereby necessary to compensate frequency offset on a received signal prior to demodulate in order to maintain good reception qualities.
FIG. 1 is a block diagram illustrating a configuration of a conventional radio communication apparatus. In addition, a QPSK modulation system is assumed as a modulation system in the following description.
In a radio communication apparatus illustrated in FIG. 1, a signal of carrier frequency received at antenna 1 is converted into a baseband signal at receiving RF section 2. An in-phase component and a quadrature component of the baseband signal are next converted into digital signals respectively at A/D converter 3 and A/D converter 4, subjected to compensation at compensator 5 and then demodulated at differential coherent detector 6 by differential coherent detection.
The demodulated signal is next decoded at decoder 7 to be a received message. Further, the demodulated signal is output to frequency offset estimator 8 and subjected to phase comparison with a known symbol.
FIG. 2 is an operation explanation diagram to explain compensation of frequency offset in a radio communication apparatus in the conventional system. A signal transmitted as (1,1) is plotted at point A under an ideal condition. However, the signal is actually plotted at point B different from point A because the phase rotates by frequency offset.
A phase rotation amount is calculated at frequency offset estimator 8, and a frequency offset amount is estimated as xcex81 in FIG. 2.
The estimated frequency offset amount is returned to compensator 5 to be used in compensation of next slot. In other words, the digital signals next converted at A/D converter 3 and A/D converter 4 are compensated by frequency offset amount (xcex81)at compensator 5 and output to differential coherent detector 6.
As described above, the conventional radio communication apparatus estimates a frequency offset amount by phase comparison with a known symbol and improves reception qualities by performing the compensation with this frequency offset amount.
However, in the above-described conventional radio communication apparatus, since the case where an interference station is present is not considered adequately, it is impossible to estimate a correct frequency offset under the condition where an interference station is present.
For example, as illustrated in an operation explanation diagram in FIG. 3, a signal transmitted as (1,1), which is plotted at point A under an ideal condition, is plotted at point C because the phase rotates by frequency offset amount xcex81 and interference component phase amount xcex82.
In this case, when an offset estimation is performed using a signal having an interference component by the above-described method, the offset estimation includes not only frequency offset amount xcex81 that should be only estimated but also phase rotation amount xcex82, i.e., the added result xcex81+xcex82 is estimated as a frequency offset amount. Therefore reception qualities are deteriorated when the compensation is performed under such a condition.
The first object of the present invention is to provide a radio communication apparatus and radio communication method capable of correctly estimating frequency offset to compensate even when an interference signal is present.
This object is achieved by eliminating frequency offset contained in a digital-converted received signal to demodulate, and estimating the frequency offset using the demodulated signal and a known symbol while decoding the demodulated signal.
The second object of the present invention is to provide a radio communication apparatus and radio communication method capable of decreasing a calculation amount to reduce an affect due to fading and noise in the estimation of frequency offset.
This object is achieved by rearranging all signals at the first quadrant once to average and estimating frequency offset using the averaged value.